1. Field of the Invention
The present invention relates to an apparatus and system for obtaining information related to wheel state for a vehicle.
2. Description of the Related Art
Recently, for obtaining safer travel of vehicle, there are developing a tire pressure monitoring system (TPMS) through which the information of tire pressure, temperature, and the like is transmitted wirelessly to vehicle body to inform to a driver. In such tire pressure monitoring systems, a wheel state detection unit for detecting the wheel state, such as a tire pressure and the like, is mounted in each wheel. The information monitored with the wheel state detection unit is transmitted wirelessly to the vehicle body.
With respect to this kind of monitoring system, Japanese patent Laid-open publication No. 2003-267011 describes a tire pressure alarm device that determines the tire pressure state based on a comparison between the monitored tire pressure and a threshold tire pressure, and alarms to a driver. In the technology described in the Japanese Patent Application Publication No. JP-A-2003-267011, it the tire specification changes, the tire pressure is appropriately adjusted to according to the new tire specification and the adjusted tire pressure is stored as the proper tire pressure, and a new threshold value for determining whether the tire is properly inflated is set based on the stored tire pressure.
In a tire pressure monitoring system, a wheel-state detecting unit is provided with a valve stem and is mounted in a wheel by fixing the valve stem to a wheel rim. In JP-A-2003-267011, the threshold tire pressure is set while the vehicle is stopped. However, a receiver on the vehicle body may have difficulty receiving the air pressure information because the signal strength is less than optimal due to the orientation of the wheel when the vehicle is stopped. In this case, even if the tire specification changes, the information of air pressure cannot be received while the vehicle is stopped. The threshold tire pressure cannot be changed, so that it is disadvantageous. Additionally, when wheels rotate to increase the signal strength improve the electromagnetic wave environment, the receiver on the vehicle body is able to receive the air pressure information the rotating wheels. However, in this case, it is possible that the air pressure information obtained is not accurate because the temperature inside air chamber of the tire changes.